(a) Field of the Invention
This invention relates to lubricants for powder metallurgy and to the manufacture and use of lubricants.
More particularly the lubricant comprises a microcapsule comprising a core of a liquid lubricant enclosed by a solid shell.
(B) Description of Prior Art
Powdered metals, for example, powdered iron, are used to make small, fairly intricate parts, for example, gears. The fabrication of such metallic parts by powdered metal technology involves the following steps:
(A) THE POWDERED METAL IS BLENDED WITH A LUBRICANT AND OTHER ADDITIVES TO FORM A MIXTURE,
(B) THE MIXTURE IS POURED INTO A MOULD,
(C) THE MIXTURE IS COMPACTED IN THE MOULD TO FORM A PART USING A HIGH PRESSURE, USUALLY OF THE ORDER OF 30 TONS PER SQUARE INCH,
(D) AFTER COMPACTION THE PART IS EJECTED FROM THE MOULD,
(E) THE EJECTED PART IS SUBJECTED TO A HIGH TEMPERATURE TO DECOMPOSE AND REMOVE THE LUBRICANT,
(F) THE PART IS HEATED TO A HIGHER TEMPERATURE TO CAUSE ALL THE PARTICLES OF METAL IN THE PART TO SINTER TOGETHER AND
(G) THE PART IS COOLED, AFTER WHICH IT IS READY FOR USE.
Commonly used lubricants include zinc stearate and lithium stearate.
The lubricant is added to the powdered metal for several reasons; it increases the bulk density of the uncompacted powdered metal. This means that the moulds can be shallower, for a given thickness of the final part. The bulk density is generally referred to as the "apparent density".
The lubricant allows the compacting pressure to be reduced to attain a specified density before sintering. This is very important because it means that for a given pressure a larger part can be made. Because of the very large pressures required to compact powdered metal, only relatively small parts are made. The density of the compacted part is called the "green density".
The ejection force to remove the compacted part from the mould is much lower when a lubricant is present and this lower force results in less mould wear.
Unfortunately, the lubricant also has a few adverse effects; it often reduces the flow rate of the powdered metal and therefore the rate at which a mould can be filled; it reduces the strength of the compacted part, referred to as the "green strength"; further, it can cause an unattractive surface finish on the sintered part. Zinc stearate is commonly used as a lubricant and slowly deposits a thin coating of zinc and zinc oxide on the walls of the oven used to burn off the lubricant or on the walls of the sintering oven.
This last disadvantage is often serious, and because of it a wax is sometimes used instead of zinc stearate. The most commonly used wax is ethylenebisstearamide; however, it is not as good a lubricant as zinc stearate, especially with regard to compressibility, i.e., it gives a lower green density for a given compacting pressure. It can only provide the same compressibility as zinc stearate if it is ground to a very fine powder using a special grinding mill which is expensive and consumes a great deal of energy.
A further disadvantage to customarily used lubricants is that they are dusty.